Environmental dosimeter of the thermo-luminescent type

ABSTRACT

A dosimeter for accurately monitoring normally low-energy radiation includes thermoluminescent CaF phosphors enclosed within a tantalum capsule. The tantalum acts as a filter to weaken the measured dose due to photons having energies below about 0.2 MeV. Tantalum end caps are maintained on the capsule body by a polyolefin sheath formed from heat-contractable tubing. After exposing the dosimeter to environmental radiation, it is placed in a shielded chamber for about 24 hours and subsequently annealed at about 80*C. to release radiation energy accumulated in low-temperature traps. The dosimeter is then disassembled and the phosphors photometrically read at temperatures about 80*C. to determine the absorbed radiation dose.

[ 51 Jan. 29, 1974 ENVIRONMENTAL DOSIMETER 0F Tim THERMO-LUMINESCENTTYPE Inventors: Fred N. Eichner, Kennewick; Leo F.

Kocher, Richland, both of Wash.

The United States of America as represented by the United States AtomicEnergy Commission, Washington, DC.

Filed: May 18, 1973 Appl. No.: 361,757

Assignee:

US. Cl 250/484, 250/337 Int. Cl. G01t 1/11 Field of Search 250/337, 484

References Cited UNITED STATES PATENTS 1/1966 Durkee et al. 250/4841/1971 Attix et al 250/484 Primary Examiner-Archie R. Borchelt Attorney,Agent, or Firm-John A. Horan; Arthur Churm; Hugh Glenn [57] ABSTRACT Adosimeter for accurately monitoring normally lowenergy radiationincludes thermoluminescent CaF phosphors enclosed within a tantalumcapsule. The tantalum acts as a filter to weaken the measured dose dueto photons having energies below about 0.2 MeV. Tantalum end caps aremaintained on the capsule body by a polyolefin sheath formed fromheatcontractable tubing. After exposing the dosimeter to environmentalradiation, it is placed in a shielded chamber for about 24 hours andsubsequently annealed at about 80C. to release radiation energyaccumulated in low-temperature traps. The dosimeter is then disassembledand the phosphors photometrically read at temperatures about 80C. todetermine the absorbed radiation dose.

9 Claims, 2 Drawing Figures I i// i/I/l/I V I// I// PMENTEI] JAN 2 9I974 NH E In W ENVIRONMENTAL DOSIMETER OF THE THERMO-LUMINESCENT TYPECONTRACTUAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the UNITED STATESATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to environmentaldosimeters that can be placed at various locations around a nuclearfacility to estimate the radiation dose that would be sustained by anindividual occupying that location. More particularly, the inventionrelates to dosimeters that employ thermoluminescent phosphors inmeasuring low-level radiation dose.

A thermoluminescent phosphor exposed to radiation will accumulateradiation energy by the movement of electrons from the valence band intoF centers or color centers. More simply stated, the radiation energy isstored in traps between the valence and conduction bands of the phosphoratoms. The accumulated energy is correlatable to radiation dose thatwould be absorbed by an individual at the dosimeter location and can betermed dose capability or absorbed dose. The theory of energyaccumulation in thermoluminescent phosphors is further explained inLuminescence Dosimetry, Proceedings of International Conference onLuminescence Dosimetry, Stanford University, Stanford, Calif, 1965,available as CONF-650637 from Clearinghouse for Federal Scientific andTechnical Information, National Bureau of Standards, U. S. Department ofCommerce, Springfield, Va. 22151.

Both UP and CaF phosphors have been used in this manner to measureabsorbed close, but it has been found that LiF phosphors require longexposures, for instance on the order of at least a month, inenvironments having low-radiation levels to store sufficient radiationenergy for accurate dose readings. On the other hand, CaF phosphors canaccumulate sufficient radiation for meaningful and reproducible readingsafter only 5-7 days exposure to low-radiation levels, e.g., levels thatwould produce about 2 to 5 millirems total accumulated dose over thatinterval. However, CaF phosphors greatly overrespond to low-energyradiation. It has been found that a CaF phosphor biased with dysprosiumwill give a sizable overresponse to sustained radiation below about 0.2MeV. A maximum overresponse of about 17 times the actual dose receivedhas been detected at about 30 KeV.

Various filter materials such as copper, brass, iron and lead have beenemployed in attempts to reduce this overresponse in CaF phosphors.However, no suitable filter material has previously been found. Mostmaterials have failed to selectively attenuate low-energy radiation andothers, particularly lead, which interact with the incident radiation toproduce K-fluorescent X-rays give altered dose readings.

In reading thermoluminescent phosphors, the energy stored in traps isreleased as scintillating phosphorescence by elevating the temperatureof the phosphor within a commercially-available, reader device. A readerwill normally include a closed chamber for receiving and heating thephosphor and a photoelectric device for registering the resulting lightscintillations. Readers of this type and methods of their use are moreSUMMARY OF THE INVENTION Therefore, in view of the limitations of theprior art, it is an object of the present invention to provide anuncomplicated and rugged radiation monitor for accurately determiningenvironmental radiation dose.

It is also an object to provide a dosimeter containing athermoluminescent phosphor which will register an effective amount ofradiation to allow reproducible readings after a reasonable exposuretime, but yet a dosimeter which will not greatly overrespond to lowenergy photons.

It is a further object to provide an accurate method I for determiningenvironmental dose with a thermoluminescent phosphor whereininconsistencies resulting from fade at low temperatures are minimized.

In accordance with the present invention, a dosimeter includes a CaFthermoluminescent phosphor enclosed within a capsule. The capsule haswalls including an effective thickness of tantalum to attenuatelowenergy photons and thereby flatten the response of the phosphor inrespect to absorbed dose when the dosimeter is employed to monitorenvironmental radiation.

After the dosimeter has been exposed to radiation energy, it is enclosedwithin a radiation-shielded chamher for about 24 hours and then annealedat C. for about 20 minutes to release the energy from the lowtemperaturetraps within the phosphor. Subsequently, the phosphor is heated totemperatures between 80C. and 260C. to release and detect, as absorbeddose, the energy stored in high-temperature traps. By omitting thelow-temperature traps from the dose determinations, the effect of fadecan be minimized to give more consistent readings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustratedin the accompanying drawings wherein:

FIG. 1 is a cross section of an environmental dosimeter; and

FIG. 2 is a graph with one logarithmic scale showing the relativeresponse, in respect to administered dose, of CaF zDy phosphors atvarious energy levels for unshielded phosphors and for phosphorsshielded with tantalum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, anassembled dosimeter is shown with an array 111 of individualthermoluminescent phosphors 13. Each phosphor 13 includes acalciumfluoride salt doped with a minor portion by weight of dysprosiumor other suitable impurity such as manganese. By employing a pluralityof phosphors as shown, each may be individually read and the readingscompared.

The individual phosphors 13 are maintained in an axial array within afoamed polyurethane or other plastic spacer 15. Spacer 115 iscylindrical in shape with an elongated, slotted opening 17 sized tosnugly receive phosphors 13 in an axial array 11. Opening 17 can beformed by slitting the spacer to axial depth for a suitable distancealong its length. The phosphors are then tucked into the opening andpositioned as desired. Each phosphor 13 is shown separated from adjacentphosphors by small spaces 16 which may be eliminated to increase thenumber of phosphors carried within the dosimeter.

Spacer l5 and phosphors 13 are enclosed within a tantalum capsule 19.The capsule includes a tubular body portion 21 and two end closures 23aand 23b positioned over each end. One end surface of spacer 15 is pastedor otherwise bonded to the adjacent end closure shown as 230 to allowconvenient assembly and removal of the phosphors 13 and spacer 15 fromwithin capsule 19.

The walls of capsule 19 are of sufficient thickness of tantalum toselectively attenuate low-energy photons that would otherwise contributea disproportionate amount to the dosimeter reading. It has been foundthat a wall thickness of 250-300 microns of tantalum will effectivelyattenuate photons below about 200 KeV to give a flat response, that is aresponse with minimal energy dependence, down to about 50 KeV where thephosphors begin to underrespond. However, absorbed dose attributable tothese low-energy photons, particularly of about 30 KeV or less, isrelatively small in comparison with that produced by photons of higherenergy.

An alternate form of capsule 19 can be provided through use of abimetallic wall structure to modify the filtering characteristics of thecapsule walls. For example, a wall of about 250 microns thickness oftantalum bonded to about 50 microns thickness of another metal such aslead could be employed. However, a single layer of tantalum will bepreferred where structural simplicity and minimization of secondaryradiation from the capsule materials becomes of significant importance.

The end closures 23a and 23b of capsule 19 are held in place and thedosimeter sealed against dust, moisture, and ultraviolet as well asvisible light by a polymerized hydrocarbon sheath 25. This sheath is alength of opaque heat-contractable tubing originally sized to slip overcapsule 19 but capable of assuming a tight fit over the capsule onapplication of heat. As examples, an opaque tubing of polyethylene,polypropylene or various other polyolefin plastics can be selected foruse in forming sheath 25.

Sheath 25 is shown disposed over capsule 19 in a manner to be flush withone end of the capsule body 21 and the outwardly disposed surface of endcap 23a. The opposite end of sheath 25 overlaps the capsule body 19 andclosure 23b. This end of the sheath is radially contracted, as a resultof heating at, for instance, 125C. for about l5-20 minutes, to form anend covering 27 over part of the outer surface of end closure 23b. Endcovering 27 thus tightly secures closure 23b to the capsule body 21.

At the opposite end of the dosimeter from end covering 27, an end cap 29of similar composition to sheath 25 is fitted over the sheath opening.The cap 29 is secured onto the dosimeter by bindingly overlapping theside portions of sheath 25 as shown. its inner surface is bonded to endclosure 23a to form an integral unit of the cap 29, closure 23a andspacer 15. Alternatively, a

tapered end plug can be employed if sheath 25 is of sufficient length tooverlap capsule 19 at both ends. An end covering as formed at 27 isprevented by inserting the tapered plug into one end of sheath 25 whileheatshrinking the sheath onto the capsule.

Through use of the heat-contractable sheath 25 and end cap 29 to securethe capsule together, it is unnecessary to weld or solder the tantalumparts. These operations, involving fusion of materials, are not onlydifficult to perform with tantalum, but also tend to alter the wallthickness. It is particularly important that the dosimeter of thepresent invention be provided with capsules of precise wall thickness toeffectively attenuate low-energy photons and thereby minimizeoverresponse. In addition, it is desirable that the variations in wallthickness between capsules be small to provide consistent overalldosimetric results.

At thus described, the dosimeter is readily disassembled into twointegral units. The first is glued or otherwise bonded together andcomprises end cap 29, end closure 23a, foamed spacer 15 and the array 11of thermoluminescent phosphors 13. The other or second unit includes thebody portion 21 of capsule 19, closure 23b and sheath 25 as heat-shrunkover the capsule parts. The two units are easily assembled as shown inthe drawing with spacer 15 inserted into capsule 19 or disassembled toallow removal of the thermoluminescent phosphors for reading orreplacement.

In preparing CaF :Dy thermoluminescent phosphors for use, undamagedphosphors are washed with an organic solvent and annealed at about 400C.for about 1 hour followed by annealing at l00C. for 2 hours. Theannealing and cooling are carried out in shielded ovens. During transferbetween ovens and subsequent assembling of the dosimeters, exposure toultraviolet light from, for instance, fluorescent sources and sunlightis avoided. The assembled dosimeters are stored in a shielded vault orcontainer until they are required for use. It is preferred that thestorage time be minimized to avoid accumulation of cosmic-ray dose priorto use.

The dosimeters are exposed to environmental radiation for at least 5days, but ordinarily an exposure of about 10 days to two weeks is usedto attain a sufficient, absorbed dose. During exposure, a dosimeter iscontained within a plastic bag of, for instance, polyethylene orpolypropylene and suspended by a cord from a cantilevered or otheroverhead support. The bag containing the dosimeter is suspended at alocation about one meter above the ground to approximate the gonadallevel of an individual. It is also held at least onehalf meter fromnearby structural members to minimize effects from reflected andsecondary radiation. Use of plastics such as polytetrafluoroethylene andpolyethylene in the supporting members can further reduce reflected andsecondary radiation at the dosimeter.

Following exposure, the dosimeters are maintained in. a shielded vaultfor about 24 hours to allow the initial, rapid fading of the phosphorsto occur. Then the unopened dosimeters are given a second annealing atC. for about 20 minutes prior to reading. This postexposure annealingremoves the radiation energy stored in the low-temperature traps of thephosphors and further ensures against error resulting from fading of thethermoluminescent response with time. It will be understood that,although the above delay and annealing times as well as temperatures aregiven in approximate values, accuracy is enhanced when all of thedosimeters are treated identically and in an identical manner to thatused in calibrating the thermoluminescent reading to absorbed radiationdose.

After annealing, the dosimeters are disassembled in subdued light andthe phosphors are read at temperatures between 80C. and 260C. in acommerciallyavailable reader for thermoluminescent dosimeters. Forexample, a reader as described in Cameron et al., ThermoluminescentDosimetry, cited above, would be suitable for this purpose. Eachphosphor is read at least twice the first to obtain a gross reading andthe last to obtain the response of a blank phosphor. It is occasionallydesirable to introduce an intermediate reading after the phosphor hascooled from the initial one to verify a complete readout of accumulatedradiation energy. The net radiation energy absorbed or dose isrepresented by the difference between the first and the last reading. Aswith most measurements of this type, the net reading is related toactual dose in milliroentgens by a conversion or calibration functionthat is normally represented in graph form. The function is obtained byexposing similar dosimeters to known amounts ofgamma radiation from, forinstance, cobalt- 60 or radium-226 sources.

A correction can be made for the amount of fade which occurs from theintial to the end portion of the exposure period. This correction isbased on exponential fade of accumulated dose with estimated orempirically determined fade constants. The dose rate is given asfollows:

where: I

D is the actual dose rate in microroentgens/hour R is the dose readingin microroentgens T is the exposure time in days.

The readings obtained with the presently described dosimeters arecompared with similar readings obtained from the bare phosphors. Theresults are plotted in the two curves presented in the graph of FIG. 2.The upper curve represents reading of the bare phosphors, while thelower curve is composed of readings obtained from the assembleddosimeters including a tantalum capsule having 250 micron thick walls.The readings in both cases are presented in respect to the theoreticaldose calculated from the gamma energy spectrum of a known source, i. e.,a radium-226 gamma source, to give a relative response. The relativeresponse is plotted against photon energy and ideally should have avalue of 1.0 at all energy levels. It can be seen from the drawing thata flat response at about a relative response of 1 can be obtained downto about 50 KeV with the tantalum-shielded dosimeter. Relative responsesbelow 1.0 down to 0.3 are obtained at energy levels of 50 KeV to 30 KeV.The absorbed radiation dose produced by photons having energy below 30KeV becomes sufficiently low and can be disregarded, as it will normallybe only a small fraction of the total dose. The upper curve in FIG. 2illustrates the substantial overresponse in respect to dose produced byreadings taken from the unshielded CaF zDy phosphors.

It will be seen that the present invention provides a rugged anduncomplicated dosimeter for measuring environmental radiation along withan improved method for reading thermoluminescent phosphors. A thin-walltantalum capsule encloses CaF :Dy thermoluminescent phosphors andthereby sufficiently attenuates lowenergy photon radiation to avoidoverresponse that 5 would otherwise occur. By using an opaquepolymerized-hydrocarbon sheath that can be sufficiently contracted bythe application of heat to hold the end caps of the capsule onto thebody, the welding of tantalum parts is avoided in assembling thedosimeter. Conse- 10 quently, dosimeters having tantalum capsules ofconsistent thickness are provided with accompanying improved consistencyin dosimetric readings. Other improvements in the method of the presentinvention allow inconsistencies that would result from fade in 5low-temperature traps to be minimized.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A dosimeter for measuring environmental radiation dose with minimaloverresponse to low-energy photons comprising:

A. a CaF thermoluminescent phosphor; and

B. a capsule enclosing said phosphor, said capsule having wallsincluding an effective thickness of tantalum to attenuate low-energyphotons and thereby minimize disproportionate dosimetric response ofsaid CaF thermoluminescent phosphor.

2. The dosimeter of claim 1 wherein said capsule includes a tubularportion with end closures, and wherein heat-contractable means areprovided to secure said end closures onto said tubular portion.

3. The dosimeter of claim 2 wherein said heatcontractable meanscomprises a polyolefin sheath disposed in a shrink fit with one end ofsaid sheath positioned flush with a first end closure of said capsule,and the opposite end of said sheath overlapping the opposite end and asecond end closure of said capsule, said overlapping end of said sheathbeing radially contracted to tightly fit over said second end closure,and further comprises a removable polyolefin cap snugly disposed overthe flush end of said polyolefin sheath and said first end closure.

4. The dosimeter of claim 1 wherein said phosphor is maintained at acoaxial location within said capsule by the interposition of foamedmaterial to envelop said phosphor withinsaid capsule.

5. The dosimeter of claim 4 wherein said foamed material is polyurethanefoam.

6. The dosimeter of claim 1 wherein said capsule walls include atantalum thickness of 250 to 300 microns.

7. A method of determining absorbed radiation dose in the dosimeter ofclaim 1 comprising:

A. maintaining said capsule with phosphor within a totally enclosed,radiation-shielded chamber for about 24 hours;

B. annealing said phosphor at about 80C. for about 20 minutes; and

C. photometrically reading said phosphor for sustained radiation attermperatures between C. and 260C.

8. A method of determining environmental radiation 5 dose with adosimeter including a CaF thermoluminescent phosphor comprising:

A. exposing said dosimeter to environmental radiation;

B. maintaining said dosimeter within a totallytained radiation attemperatures above 80C.

enclosed, radiation-shielded chamber for about 24 9. The method of claim8 wherein said thermolumihours; nescent phosphor is preannealed at 400C.for about 1 C. annealing said phosphor at about 80C. for about hour andmaintained at 100C. for about 2 hours prior minutes; and 5 to saidradiation exposure.

D. photometrically reading said phosphor for sus-

2. The dosimeter of claim 1 wherein said capsule includes a tubularportion with end closures, and wherein heat-contractable means areprovided to secure said end closures onto said tubular portion.
 3. Thedosimeter of claim 2 wherein said heat-contractable means comprises apolyolefin sheath disposed in a shrink fit with onE end of said sheathpositioned flush with a first end closure of said capsule, and theopposite end of said sheath overlapping the opposite end and a secondend closure of said capsule, said overlapping end of said sheath beingradially contracted to tightly fit over said second end closure, andfurther comprises a removable polyolefin cap snugly disposed over theflush end of said polyolefin sheath and said first end closure.
 4. Thedosimeter of claim 1 wherein said phosphor is maintained at a coaxiallocation within said capsule by the interposition of foamed material toenvelop said phosphor within said capsule.
 5. The dosimeter of claim 4wherein said foamed material is polyurethane foam.
 6. The dosimeter ofclaim 1 wherein said capsule walls include a tantalum thickness of 250to 300 microns.
 7. A method of determining absorbed radiation dose inthe dosimeter of claim 1 comprising: A. maintaining said capsule withphosphor within a totally enclosed, radiation-shielded chamber for about24 hours; B. annealing said phosphor at about 80*C. for about 20minutes; and C. photometrically reading said phosphor for sustainedradiation at termperatures between 80*C. and 260*C.
 8. A method ofdetermining environmental radiation dose with a dosimeter including aCaF2 thermoluminescent phosphor comprising: A. exposing said dosimeterto environmental radiation; B. maintaining said dosimeter within atotally-enclosed, radiation-shielded chamber for about 24 hours; C.annealing said phosphor at about 80*C. for about 20 minutes; and D.photometrically reading said phosphor for sustained radiation attemperatures above 80*C.
 9. The method of claim 8 wherein saidthermoluminescent phosphor is preannealed at 400*C. for about 1 hour andmaintained at 100*C. for about 2 hours prior to said radiation exposure.